In the process of rotary drilling a well, drilling fluid, or mud, is circulated down the rotating drill pipe, through the bit, and up the annular space between the pipe and the formation or steel casing, to the surface. The drilling fluid performs different functions such as removal of cuttings from the bottom of the hole to the surface, to suspend cuttings and weighting material when the circulation is interrupted, control subsurface pressure, isolate the fluids from the formation by providing sufficient hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, cool and lubricate the drill string and bit, maximize penetration rate, etc.
The required functions can be achieved by a wide range of fluids composed of various combinations of solids, liquids and gases and classified according to the constitution of the continuous phase mainly in two groupings: aqueous drilling fluids, and oil-based drilling fluids. In drilling water-sensitive zones such as reactive shales, production formations, or where bottom hole temperature conditions are severe or where corrosion is a major problem, oil-based drilling fluids are preferred.
Oil-based drilling fluids typically contain oil-soluble surfactants that facilitate the incorporation of water-wet clay or non-clay formation minerals, and hence enable such minerals to be transported to surface equipment for removal from circulation before the fluid returns to the drill pipe and the drill bit. The largest formation particles are rock cuttings, the size typically larger than 0.1 to 0.2 mm, removed by shale-shaker screens at the surface. Smaller particles, typically larger than about 5 μm, will pass through the screens, and must be removed by centrifuge or other means.
Oil-based drilling fluids have been used for many years, and their application is expected to increase, partly owing to their several advantages over water based drilling fluids, but also owing to their ability to be re-used and recycled, so minimizing their loss and their environmental impact.
As mentioned above, during drilling, formation particles become incorporated into the drilling fluid. Unless these are removed, they eventually alter the fluid's properties, particularly the Theological parameters, out of the acceptable range. However, formation particles that are less than about 5 to 7 μm in size are more difficult to remove than larger particles using conventional separation means such as shale shakers. These low gravity solids can build up in a mud system, causing inefficient drilling problems such as drill pipe sticking, increased pipe torque, and other high viscosity issues.
While low gravity solids may be removed from drilling fluids using mechanical means such as a centrifuge, it has been found that longer run-times are required to remove the colloidal particles, if the low gravity solids can be removed at all. Thus, there is a need for an apparatus that can be used with traditional solids separation equipment to reduce the run-time required to remove low gravity solids. Further, it would be an improvement in the art to have an apparatus that can be utilized both on active drilling projects to facilitate solids control equipment efficiency as well as by mud plants in reclaiming and/or reconditioning mud returned from field operations.